1. Technical Field
The present invention relates generally to laser alignment targets and, more particularly, to a laser alignment target with structure to compensate for loss of contrast.
2. Related Art
Semiconductor integrated circuit devices typically contain fuses which are used, for example, to invoke redundant circuit elements, create electronic chip identification or xe2x80x9ctrimxe2x80x9d resonant circuits to achieve desired device performance. FIG. 1 illustrates a cross section of an exemplary back-end-of-line (BEOL) (wiring and insulators exposed) of a semiconductor integrated circuit device 2. Dielectric layers 4, 6 and 8, such as transparent silicon nitride, silicon dioxide or silicon oxide, contain wiring (not shown), e.g., copper, aluminum, etc. Layer 4 is in contact with a substrate 10, which is, for example, silicon containing doped regions to form transistors, etc. Uppermost dielectric layer 12 contains a wiring layer that has, as part of its design, at least one metal fuse 14. Element 16 is part of the wiring layer that contains fuse 14 and is used to connect to subsequent wiring layers or to the environment beyond the die. It is included to provide one example of how the fusing wiring layer would be connected to other features, for example, a metal layer element 18, which might be another wiring layer or a wire bond pad.
In operation, a laser fusing tool 20 is typically used to selectively delete fuses 14, or parts thereof, by illuminating the fuse wiring segment with laser radiation. The illumination causes heating/ablating of the wiring segment. To effectively heat/ablate the fuse, the laser beam must accurately illuminate the wiring segment. Accordingly, the laser beam must be aligned to the fuse wiring segment prior to illumination. Alignment is typically achieved by scanning the laser beam across both xe2x80x9cXxe2x80x9d and xe2x80x9cYxe2x80x9d direction of an alignment target 22. The difference in reflected energy over target 22 and an adjacent field/surface 24 is used to determine the exact position of the target. Typically, the area above target 22 is highly reflective while adjacent surface(s) 24 has much lower reflectivity. The alignment target specified by the laser fusing tool vendors is commonly a reflective xe2x80x9cLxe2x80x9d or xe2x80x9cTxe2x80x9d shape, and multiple targets 22 are commonly provided. Once an xe2x80x9coriginxe2x80x9d is established, the laser beam can be offset by the required xe2x80x9cxxe2x80x9d and xe2x80x9cyxe2x80x9d distances to have the center of the beam illuminate the center of fuse 14 for deleting.
One obstacle to assuring that an alignment target can be ascertained is lack of contrast between the target and the adjacent, surrounding films stack. Lack of contrast can be the result of a number of issues, including residuals over or under the target and its adjacent films regions. Examples of residuals include both metallic and nonmetallic film fragments that result from faulty chemical-mechanical polish or etch back removal processes. The residuals introduce noise on the reflected light signal and in severe cases, there is sufficient residual to substantially remove any contrast between the target and adjacent regions.
One proposed remedy to the above problem has been to focus on removal and/or reduction of the residuals by additional processing. However, these remedies do not provide an adequate solution, unsatisfactorily add costs and/or introduce other undesired variation in the device structure. For example, introductions of a xe2x80x9cclean upxe2x80x9d etch after chemical-mechanical polish can introduce roughness in the dielectric covering the alignment target and surrounding area, and that roughness and associated changes in films stack optical thickness can degrade reflected light signal to noise ratios.
Complex film stacks that include a number of layers above the target can also destroy contrast between the alignment target and the adjacent surface. For example, referring to FIG. 1, passivation layer 26 is desirable if the wiring layer includes a non-self passivation metal such as copper (Cu). Layer 26 is over fuse 14 and alignment target 20 and, hence, must be traversed by the laser beam during scanning. Where a number of these layers are provided, further contrast problems are created. In addition, where the thickness of layers varies within a stack, further contrast difficulties can result. This is especially the case where short wavelength ultraviolet laser light (e.g.,  less than 400 nm) is used. This short wavelength light is desirable because laser spot size can be reduced relative to those of longer wavelengths. Using short wavelength light, therefore, allows for more precise ablating of fuses and smaller fuse sizes. Unfortunately, small variations of optical path length created by additional layers have a large effect on reflected energy with short wavelength lasers. It is not uncommon, in the short wavelength range, to find that insufficient contrast exists to accomplish determination of the exact position of the target.
In view of the foregoing, there is a need in the art for an alignment target capable of providing contrast despite the presence of complex film stacks and/or metal residuals.
A first aspect of the invention is directed to a laser alignment target comprising: a surface that is out of plane with and has substantially the same first reflectivity as an adjacent surface of the semiconductor device; and a sidewall having a second reflectivity different than the first reflectivity.
A second aspect of the invention provides a semiconductor device comprising: a plurality of fuses provided on a first level of interconnect; and an alignment target including: a surface that is out of plane with and has substantially the same first reflectivity as an adjacent surface; and a sidewall having a reflectivity different than the first reflectivity.
A third aspect of the invention is directed to a method of creating a laser alignment target, the method comprising: creating a surface having substantially the same first reflectivity as an adjacent surface; and forming sidewalls between the surface and the adjacent surface having a second reflectivity different than the first reflectivity.
The foregoing and other features of the invention will be apparent from the following more particular description of embodiments of the invention.